Beams of high intensity ultraviolet (UV) light are useful for curing polymers in coatings, inks, adhesives and the like, and for other purposes. A known reliable source of UV light at good power is the mercury vapor lamp. Almost all of the world's artificial lighting comes from mercury lamps, mainly due to their efficiency and low cost. For the same reason, mercury lamps have been the staple of the UV curing industry for many decades.
Mercury lamps powered by heavy transformer ballasts, once used only in factory installations, are now appearing in on-site UV curing applications. New waterborne and dual-cure UV coating technologies are also expanding UV curing from factory to the jobsite. Bathtubs, countertops, floors, walls, etc., are now finished and re-finished quickly in the field, with conventional curing equipment. Lighter, simpler, and more practical needs along with recent economic and environmental concerns are driving the demand for more portable lower cost UV systems. Lower cost and portability will drive the equipment side of the UV curing market, while appearance, durability, and applicability will dominate coatings.
A ballast circuit is used with gas or vapor discharge lamps to control their power usage and prevent runaway overloading of the power supply. Ballasts can include resistors, capacitors, inductors, transformers or a combination of these, as well as electronic circuits where the power to the lamp is controlled by high frequency switching techniques. Today the most widely used and heaviest part of the ballast for the lamps in UV curing equipment is the transformer.
While ordinary fluorescent lamps for lighting need only small ballasts due to their low power, typical UV curing lamps use tens or hundreds of watts per centimeter of length, requiring high-power ballasts rated at 1 to 40 kW. Magnetic transformer ballasts use iron cores. Their weight in kilograms (kg) is approximately 5.4 P+18, where P is the power in kW. The most common power source is the 120 volt 15 Amp residential outlet delivering a maximum of 1.8 kW. Other electrical outlets deliver 220 volt at 15, 20, 30 and 50 amps, respectively delivering a maximum of 3.3 kW, 4.4 kW, 6.6 kW, and 11 kW of power to the lamp. Typical UV lamps operating at these power levels require magnetic ballasts that will weigh approximately 28, 36, 42, 54, and 77 kg, respectively. The entire system, however, can weigh more. This limits the portability of the UV curing equipment.
Electronic ballasts offer an alternative to the heavy magnetic ballast. 3.5 kW compact electronic ballasts are now available in the marketplace weighing less than 7 kg, but their cost is significantly higher than their magnetic ballast counterparts. Resonant and hybrid technologies, offer equipment at more moderate weights and prices.
In a prior handheld UV curing device of the present inventor, the electrical ballast is a resistance wire (e.g., of Nichrome®). The resistance wire also doubles as thermal ballast for the lamp. Air from a fan is blown across the wire in a path that takes the air past the lamp. A curved elongated optical reflector associated with the lamp is split so that air can enter a plenum defined by the reflector wherein the lamp is mounted axially. Whenever the lamp is cold, air heated by passing over the resistive wire of the ballast heats the lamp toward its operating temperature. When the lamp temperature exceeds the temperature of the wire, the airflow then acts to cool the lamp. This arrangement tends to stabilize the lamp's thermal performance, and since the voltage across an electrical discharge is dependent upon the gas pressure and thus upon the lamp's temperature, the arrangement also stabilizes the lamp's electrical performance.
For UV outputs of over 40 W/cm with beam widths up to 12 cm and at a curing distance of 3 to 5 cm, large housings are used to provide room for the UV lamp, ballast circuitry, and any cooling structures. What is needed is a lightweight ballast circuit for a moderate power UV lamp that can be used in a portable curing apparatus. A portable, even handheld, device would offer both speed and precision for curing of polymer coatings on surfaces of all shapes.